Concrete Volume Calculator
Calculate how much concrete you need for slabs, footings, columns, and walls. Includes cubic yards, cubic feet, and bag estimates.
Reviewed by the Calcura editorial team · Last reviewed
- Question
- How much concrete do I need?
- Answer
- For a slab: volume_ft³ = length × width × depth (all in feet). Divide by 27 to get cubic yards, the unit ready-mix trucks use. A 10' × 10' × 4" slab is 10 × 10 × 0.333 = 33.3 ft³ = 1.23 yd³. For bags: an 80 lb bag of Quikrete yields 0.60 ft³, so 33.3 ft³ ÷ 0.60 ≈ 56 bags. Round columns use volume = π × r² × height.
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- Formula
volume_yd³ = L × W × D (ft) / 27; 80-lb bag = 0.60 ft³- Caveats
- Always order 5–10% more than calculated — spillage, settling, and form inaccuracies eat volume.
- Under ~1 yd³, bags are cheaper; over that, ready-mix delivery usually wins on price and labor.
- Depth is the most common miscalculation — slab thickness must be consistent, not average.
What This Calculator Does
Calculates the volume of concrete required for slabs, footings, walls, and round columns in cubic yards (US ready-mix unit), cubic feet, and cubic meters. Translates that volume into Quikrete/Sakrete bag counts at the manufacturer-specified yields (80 lb = 0.60 ft³, 60 lb = 0.45 ft³). The bag count assumes uncompacted mixed concrete — real compaction can vary ~2%.
When To Use This Calculator
Pouring a 4-inch-thick patio or sidewalk
10×10 ft patio × 4 in thick = 33.33 ft³ = 1.23 yd³ ≈ 1.5 yd³ ordered (with ~10% overage).
Setting fence-post footings
Eight 10" diameter × 36" footings: π(5)² × 36 × 8 / 1728 = 13.1 ft³ → 22 × 80-lb bags.
Pouring a 6-inch residential driveway
20×12 ft × 6 in = 120 ft³ = 4.44 yd³. Order 5 yd³ to cover the ready-mix minimum and waste.
Building an 8-inch thick retaining wall
20 ft long × 6 ft tall × 8 in = 80 ft³ = 2.96 yd³. Round to 3.25 yd³ with 10% overage.
How It Works
For rectangular pours, volume is length × width × thickness, with thickness converted from inches to feet (÷12). Columns use V = π·r²·h where r = diameter/2 and height is converted to feet. Cubic feet divided by 27 gives cubic yards (since 1 yd³ = 3 ft × 3 ft × 3 ft = 27 ft³). Bag counts are derived from Quikrete's published yield — each 80-lb bag of mixed concrete yields 0.60 ft³ at a typical slump; 60-lb bags yield 0.45 ft³. Metric inputs are computed in meters first then converted via 1 m³ = 35.3147 ft³ (BIPM exact conversion).
Formula
slab_ft³ = L × W × (T_in/12) | column_ft³ = π × (D_in/24)² × (H_in/12) | yd³ = ft³ / 27 | bags_80lb = ceil(ft³ / 0.60)Real-World Example
Scenario
Homeowner pouring a detached-garage pad: 24 ft × 24 ft with a 4-inch slab on compacted gravel, plus a 12-inch-deep × 12-inch-wide monolithic thickened edge on all four sides. Ready-mix is $160/yd³ delivered with a 3 yd³ minimum.
Calculation
Slab: 24 × 24 × (4/12) = 192 ft³. Edge (single continuous band around perimeter, avoid double-counting corners): 4 × 24 × 1 × 1 − 4 × (1 × 1 × 1) = 92 ft³. Total 284 ft³ = 10.52 yd³. Add 10% overage = 11.58 yd³ → order 11.75 or 12 yd³. Cost at $160: ~$1,880–$1,920.
Result
Order 12 yd³ to stay above minimum and absorb form flex and subgrade variation — budget ~$1,920 for the pour itself.
Common Mistakes to Avoid
Mistake: Using the nominal slab thickness instead of compacted thickness
Solution: If the subgrade settles or dips, you'll need more concrete than the nominal spec shows. Add 5–10% specifically for subgrade variation, not only for waste.
Mistake: Forgetting to convert thickness from inches to feet
Solution: A 10×10×4 pour isn't 400 ft³ — thickness is 4 inches = 0.333 ft. Always divide inch dimensions by 12 before multiplying.
Mistake: Double-counting corners on perimeter footings
Solution: When the footing wraps a rectangular foundation, subtract one corner's worth of overlap per corner, or compute as (perimeter × width × depth) − 4·(width²·depth).
Mistake: Ordering exactly the calculated amount
Solution: Form flex, uneven subgrade, and spillage eat ~5–10%. Order at least 10% over for bags (returnable unopened) or round up to the nearest 0.25 yd³ for ready-mix.
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When This Isn't the Right Tool
Structural slabs with engineered rebar and PT cables
Use instead: Volume alone isn't enough — get a structural engineer's take on design thickness, rebar spacing, and mix PSI per ACI 318.
Mortar, grout, or non-concrete cementitious products
Use instead: Mortar and grout have different water/cement ratios and yields. Use the specific product's coverage table.
Lightweight or high-strength specialty mixes
Use instead: High-strength and lightweight mixes yield different volumes per dry weight. Follow the bag's stated yield, not the generic 0.60 ft³ per 80 lb.
How to Verify This Calculation
Sanity checks: a 10×10 slab at 4" thick must come out to 33.33 ft³ / 1.23 yd³ — if it's off, thickness wasn't divided by 12. A 1 ft × 1 ft × 1 ft cube = 1 ft³ exactly = 0.037 yd³. A 3 ft × 3 ft × 3 ft cube = 1 yd³.
Sources
- Quikrete product yield specifications — Manufacturer source for 80-lb = 0.60 ft³ and 60-lb = 0.45 ft³ bag yields used in this calculator.
- ACI 318 — Building Code Requirements for Structural Concrete — Industry reference for structural concrete design, mix strength, and slab thickness requirements.
- Portland Cement Association — concrete basics — Authoritative reference on mix design and typical slab thicknesses for residential applications.
- NIST Handbook 44 / NIST SP 1038 — measurement standards — Basis for the exact 1 yd³ = 27 ft³ and 1 m³ = 35.31466... ft³ conversions.
Reviewed by the Calcura editorial team · Last reviewed